Dual conversion gain gate and capacitor combination

ABSTRACT

An imaging device comprising a plurality of photosensors, a shared diffusion region for receiving charge generated by the photosensors, and a dual conversion gain element that can be selectively coupled to the shared diffusion region to increase a conversion gain of the shared diffusion region. A method of operating such an imaging device is also described, comprising resetting a shared diffusion region, sampling a reset voltage level at the shared diffusion region, transferring charge accumulated in one of a plurality of photosensors to the shared diffusion region, sampling a pixel signal voltage level at the shared diffusion region, and activating a dual conversion gain element to increase a conversion gain of the shared diffusion region.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 12/704,778,filed Feb. 12, 2010, now allowed, which is a continuation of U.S. patentapplication Ser. No. 12/230,014, filed Aug. 21, 2008, now U.S. Pat. No.7,683,306, which is a divisional application of U.S. patent applicationSer. No. 11/193,322, filed Aug. 1, 2005, now U.S. Pat. No. 7,432,540,the disclosures of which are incorporated by reference in theirentirety.

FIELD OF THE INVENTION

The present invention relates generally to digital image sensors and inparticular to dual conversion gain imagers.

BACKGROUND OF THE INVENTION

Imaging devices, including charge coupled devices (CCD) andcomplementary metal oxide semiconductor (CMOS) imagers, are commonlyused in photo-imaging applications.

Typically, a digital imager array includes a focal plane array of pixelcells, each one of the cells including a photosensor, e.g., a photogate,photoconductor, or a photodiode. In a CMOS imager a readout circuit isconnected to each pixel cell which typically includes a source followeroutput transistor. The photosensor converts photons to electrons whichare typically transferred to a storage node, e.g., a floating diffusionregion connected to the gate of the source follower output transistor. Acharge transfer device (e.g., transistor) can be included fortransferring charge from the photosensor to the floating diffusionregion. In addition, such imager cells typically have a transistor forresetting the storage node to a predetermined charge level prior tocharge transference. The output of the source follower transistor isgated as a pixel output signal by a row select transistor.

Exemplary CMOS imaging circuits, processing steps thereof, and detaileddescriptions of the functions of various CMOS elements of an imagingcircuit are described, for example, in U.S. Pat. No. 6,140,630, U.S.Pat. No. 6,376,868, U.S. Pat. No. 6,310,366, U.S. Pat. No. 6,326,652,U.S. Pat. No. 6,204,524, and U.S. Pat. No. 6,333,205, each of which areassigned to Micron Technology, Inc. The disclosures of each of theforgoing are hereby incorporated by reference herein in their entirety.

With reference to FIGS. 1 and 2, which respectively illustrate atop-down and a cross-sectional view of a conventional CMOS pixel cell100, when incident light 187 strikes the surface of a photodiodephotosensor 120, electron/hole pairs are generated in the p-n junctionof the photodiode photosensor 120 (represented at the boundary of n−accumulation region 122 and p+ surface layer 123). The generatedelectrons (photo-charges) are collected in the n-type accumulationregion 122 of the photosensor 120. The photo-charges move from theinitial charge accumulation region 122 to a floating diffusion region110 via a transfer transistor 106. The charge at the floating diffusionregion 110 is typically converted to a pixel output voltage by a sourcefollower transistor 108 and subsequently output on a column output line111 via a row select transistor 109.

Conventional CMOS imager designs, such as the one shown in FIG. 1 forpixel cell 100, provide approximately a fifty percent fill factor,meaning only half of the cell 100 is dedicated to converting light tocharge carriers. As shown, only a small portion of the cell 100comprises a photosensor 120 (e.g., photodiode). The remainder of thepixel cell 100 includes isolation regions 102, shown as STI regions in asubstrate 101, the floating diffusion region 110 coupled to a transfergate 106′ of the transfer transistor 106, and source/drain regions 115for reset 107, source follower 108, and row select 109 transistorshaving respective gates 107′, 108′, and 109′. In the conventional pixelcell 100 layout, each of the reset, source follower and row selecttransistor gates 107′, 108′ and 109′ are aligned consecutively, sharingsource/drain regions 115 and requiring significant surface area for eachpixel cell 100. As scaling of pixels continues to decrease the pixel's100 total area, it becomes increasingly important to create highsensitivity photosensors that utilize a minimum amount of surface area,or to find more efficient pixel cell layouts to minimize the arearequired by non-photosensitive components of the pixel cell in order tomaintain relatively large areas for the photosensors.

In addition, conventional storage nodes, such as floating diffusionregion 110, have a limited amount of charge storage capacity. Once thiscapacity is reached, a pixel cell becomes less efficient. Specifically,the full dynamic range available for the pixel cell cannot be utilized.Once the charge storage capacity is exceeded, an undesirable phenomenon,known as “blooming” occurs, whereby the “over-capacity” charges escapeto undesirable parts of the pixel cell 100 or to adjacent pixel cells.One suggested solution for dealing with this limited charge storagecapacity is to add a capacitor which is connected to the floatingdiffusion region 110. The capacitor is used to store the additionalover-capacity charges so the charges do not flow to other areas of thecell or adjacent cells. The problem with this solution, however, is thatthe additional capacitor takes up space in the cell that could otherwisebe used to increase the size of the cell's photosensor, therebydecreasing the potential fill factor for the pixel cells and overallarray.

Accordingly, there is a need and desire to increase the fill factor andstorage capacity of imager pixel cells. Thereby, the dual conversiongain gates of the invention provide good sensitivity at low and highlight conditions.

BRIEF SUMMARY OF THE INVENTION

The present invention, in the various exemplary embodiments, provides apixel cell array architecture having a dual conversion gain. A dualconversion gain element is coupled between a floating diffusion regionand a respective storage capacitor. The dual conversion gain elementhaving a control gate switches in the capacitance of the capacitor tochange the conversion gain of the floating diffusion region from a firstconversion gain to a second conversion gain. In order to increase theefficient use of space, the dual conversion gain element gate alsofunctions as the bottom plate of the capacitor.

In one particular embodiment of the invention, a high dynamic rangetransistor is used in conjunction with a pixel cell having acapacitor-DCG gate combination; in another embodiment, adjacent pixelsshare pixel components, including the capacitor-DCG combination.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages and features of the invention willbecome more apparent from the detailed description of exemplaryembodiments provided below with reference to the accompanying drawingsin which:

FIG. 1 is a top-down view of a conventional CMOS pixel cell;

FIG. 2 is a cross-sectional view of the pixel cell of FIG. 1, takenalong line 1-1′;

FIG. 3 is a top-down view of a portion of a CMOS pixel cell arrayconstructed in accordance with an exemplary embodiment of the invention;

FIG. 4 is a circuit diagram of the exemplary embodiment depicted in FIG.3;

FIG. 5 is a timing diagram of the operation of the pixel cell array ofFIG. 3 according to an exemplary embodiment of the invention;

FIG. 6 is a top-down view of a portion of a CMOS pixel cell arrayconstructed in accordance with another exemplary embodiment of theinvention;

FIG. 7 is a circuit diagram according to another exemplary embodiment ofthe invention;

FIG. 8 is a timing diagram of the pixel cell array of FIG. 6 accordingto another exemplary embodiment of the invention;

FIG. 9 is a block diagram of a CMOS imager chip having an array of pixelcells constructed in accordance with the invention; and

FIG. 10 is a schematic diagram of a processing system employing a CMOSimager constructed in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof and show by way ofillustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized, and thatstructural, logical, and electrical changes may be made withoutdeparting from the spirit and scope of the present invention. Theprogression of processing steps described is exemplary of embodiments ofthe invention; however, the sequence of steps is not limited to that setforth herein and may be changed as is known in the art, with theexception of steps necessarily occurring in a certain order.

The terms “wafer” and “substrate,” as used herein, are to be understoodas including silicon, epitaxial, silicon-on-insulator (SOI) orsilicon-on-sapphire (SOS) technology, doped and undoped semiconductors,and other semiconductor structures. Furthermore, when reference is madeto a “wafer” or “substrate” in the following description, previousprocessing steps may have been utilized to form regions, junctions, ormaterial layers in or over the base semiconductor structure orfoundation. In addition, the semiconductor need not be silicon-based,but could be based on silicon-germanium, germanium, gallium arsenide orother semiconductors.

The term “pixel,” as used herein, refers to a photo-element unit cellcontaining a photosensor and associated transistors for convertingphotons to an electrical signal. For purposes of illustration, a smallnumber of representative pixel cells are illustrated in the figures anddescription herein; however, typically fabrication of a large pluralityof like pixel cells proceeds simultaneously. Accordingly, the followingdetailed description is not to be taken in a limiting sense, and thescope of the present invention is defined only by the appended claims.

The terms “at an angle,” “angled,” and “slanted,” as used herein are tobe interpreted as meaning at any angle, with respect to some statedreference point, that is not exactly parallel or exactly perpendicular.Accordingly, when a portion of an object and some reference point meetto form an angle that is not 0°, 90°, or 180°, the object is considered“angled,” “at an angle,” or “slanted” with respect to the referencepoint.

The invention generally relates to a dual conversion gain element gatefunctioning as the bottom plate of a storage capacitor in a pixel cell.The capacitor serves to increase the charge capacity of the pixel cellwhen needed. The dual conversion gain (DCG) gate improves the lightperformance while providing a wide dynamic range for the pixel cells.The inventive combination provides an efficient pixel array architecturethat increases the fill factor of pixel cells in the array. In oneparticular embodiment of the invention, a high dynamic range transistor(HDR) is used in conjunction with a pixel cell having a capacitor-DCGgate combination; in another embodiment, adjacent pixels share pixelcomponents, including the capacitor-DCG combination.

Referring to FIGS. 3-4, one exemplary embodiment of the combinedcapacitor-dual conversion gain element gate is illustrated in connectionwith a 4-way shared pixel circuit 450. A top down view of a substratecontaining the pixel circuit 450 is illustrated in FIG. 3. A circuitdiagram of the pixel cell 450 of FIG. 3 is illustrated in FIG. 4. Asshown in FIG. 4, the pixel circuit 450 comprises portions of four pixelcells 450 a, 450 b, 450 c, 450 d. The illustrated shared pixel circuit450 also includes a shared dual conversion gain (DCG) transistor 406,capacitor 408, reset transistor 410, source follower transistor 412, rowselect transistor 414, and floating diffusion region 400. As discussedbelow in more detail, the gate of the DCG transistor 406 also serves asone plate of the capacitor 408.

The first pixel cell 450 a includes a first photosensor 401 a(illustrated as a photodiode), and a first charge transfer transistor416 a. A first high dynamic range (HDR) transistor 418 a may be part ofthe pixel cell 450 a if desired. The first HDR transistor 418 a isconnected between the photosensor 401 a and a pixel supply voltageVAA_PIX. The gate terminal of the HDR transistor 418 a, if used, isconnected to receive a first row high dynamic range control signal HDR<0>. When the control signal HDR <0> is received, the HDR transistor 418a allows for some charge to be drained away from the photosensor 401 a.

The first transfer transistor 416 a is connected between the firstphotosensor 401 a and a shared floating diffusion region 400 and iscontrollable by a first row even transfer gate control signalTX_EVEN<0>. When the first even transfer gate control signal TX_EVEN<0>is generated, the first transfer transistor 416 a is activated whichallows charge from the first photosensor 401 a to flow to the floatingdiffusion region 400.

The second pixel cell 450 b includes a second photosensor 401 b(illustrated as a photodiode), and a second transfer transistor 416 b. Asecond HDR transistor 418 b may be part of the pixel cell 450 b ifdesired. The HDR transistor 418 b is connected between the photosensor401 b and the pixel supply voltage VAA_PIX. The gate terminal of the HDRtransistor 418 b is connected to receive the first row high dynamicrange control signal HDR <0>. When the control signal HDR <0> isreceived, the transistor 418 b allows some charge to be drained awayfrom the photosensor 401 b.

The second transfer transistor is 416 b is connected between the secondphotosensor 401 b and the shared floating diffusion region 400 and iscontrollable by a first row odd transfer gate control signal TX_ODD<0>.When the first row odd transfer gate control signal TX_ODD<0> isgenerated, the second transfer transistor 416 b is activated whichallows charge from the second photosensor 401 b to flow to the floatingdiffusion region 400.

The third pixel cell 450 e includes a third photosensor 401 c(illustrated as a photodiode), and a third transfer transistor 416 c. Athird HDR transistor 418 c may be part of the pixel cell 450 c ifdesired. The HDR transistor 418 c is connected between the photosensor401 c and the pixel supply voltage VAA_PIX. The gate terminal of thethird HDR transistor 418 c is connected to receive a second row highdynamic range control signal HDR <1>. When the control signal HDR <1> isreceived, the transistor 418 c allows for some charge to be drained awayfrom the photosensor 401 c.

The third transfer transistor is 416 c is connected between the thirdphotosensor 401 c and the shared floating diffusion region 400 and iscontrollable by a second row even row transfer gate control signalTX_EVEN<1>. When the second row even transfer gate control signalTX_EVEN<1> is generated, the third transfer transistor 416 c isactivated which allows charge from the third photosensor 401 c to flowto the floating diffusion region 400.

The fourth pixel cell 450 d includes a fourth photosensor 401 d(illustrated as a photodiode), and a fourth transfer transistor 416 d. Afourth HDR transistor 418 d may be part of the pixel cell 401 d ifdesired. The fourth HDR transistor 418 d is connected between thephotosensor 401 d and the pixel supply voltage VAA_PIX. The gateterminal of the fourth HDR transistor 418 d is connected to receive asecond row high dynamic range control signal HDR <1>. When the controlsignal HDR <1> is received, the transistor allows for some charge to bedrained away from the photosensor 401 d.

The fourth transfer transistor is 416 d is connected between the fourphotosensor 401 d and the shared floating diffusion region 400 and iscontrollable by a second odd transfer gate control signal TX_ODD<1>.When the second odd transfer gate control signal TX_ODD<1> is generated,the fourth transfer transistor 416 d is activated which allows chargefrom the fourth photosensor 401 d to flow to the floating diffusionregion 400.

The capacitor-DCG gate combination 408, 406 is shared by all four pixelcells 450 a, 450 b, 450 c, 450 d. The capacitor 408 receives a supplyvoltage from the DCG gate voltage. The floating diffusion region 400 isconnected to the drain of the DCG transistor 406. The source of the DCGtransistor 406 is connected to the plate of the capacitor 408. The fourphotosensors 401 a, 401 b, 401 c, 401 d each share a gate of thetransfer transistor 416 a, 416 b, 416 c, 416 d with adjacent pixels,however, each photosensor 401 a, 401 b, 401 c, 401 d has its owntransfer transistor 416 a, 416 b, 416 c, 416 d. A source/drain terminalof the source follower transistor is connected to the array pixel supplyvoltage VAA_PIX. The row select transistor 414 is connected between thesource follower transistor 412 and a column line. Each of the HDRtransistors 418 a, 418 b, 418 c, 418 d share a drain contact. It shouldbe noted that the HDR transistors 418 a, 418 b, 418 c, 418 d areoptional components that are not necessary to practice the invention.

One advantage of the capacitor-DCG gate combination 408, 406 is itsefficient use of space. As noted above, the DCG gate functions 406 asthe bottom plate of the capacitor 408, which reduces the need forfabricating two plates for the capacitor 408.

The reset transistor 410 is connected between array pixel supply voltageVAA_PIX and the floating diffusion region 400. The capacitor 408 isconnected between the gate terminal of the DCG transistor 406 and thesource of the DCG transistor 406. The DCG transistor 406 has the gateconnected to a dual conversion gain control signal DCG. When the dualconversion gain signal DCG is generated, the DCG transistor 406 isactivated, which connects the storage capacitance of the capacitor 408to the floating diffusion region 400. This increases the storagecapability of the floating diffusion region 400, which is desirable andmitigates floating diffusion leakage problems. As set forth above, thepixels cells 450 a, 450 b, 450 c, 450 d share the source followertransistor 412, reset transistor 410, and row select transistor 414.Each transfer gate 416 a, 416 b, 416 c, 416 d transfers charge to theshared floating diffusion 400 region independently of the otherphotosensors 401 a, 401 b, 401 c, 401 d. This shared pixel cell 450arrangement allows for each photosensor 401 a, 401 b, 401 c, 401 d toremain large due to the sharing of pixel components 400, 414, 410, 412,418 and the combined DCG gate and capacitor plate.

In the exemplary embodiment described above, at least a portion of thetransfer gate 416 (FIG. 3) has an angled edge with respect to thephotosensors 401 a, 401 b, 401 c, 401 d. For example, with reference toFIG. 3, the sides of the transfer gates 416 a, 416 b, 416 c, 416 d areshown as being slanted, which allows for a larger photosensor 401 a, 401b, 401 c, 401 d. This angled geometry of the transfer gates 416 a, 416b, 416 c, 416 d allows for an efficient layout of the transfer gates. Inaddition, this angled layout is also beneficial in maximizing the fillfactor of the pixel cell 450, by maximizing the area of the photosensor401 a, 401 b, 401 c, 401 d for each pixel cell 450 a, 450 b, 450 c, 450d.

FIG. 5 is a timing diagram illustrating one exemplary operation of aportion of the four-way pixel cell 450 illustrated in FIGS. 3-4. Forclarity purposes only, the timing diagram illustrates the operation ofthe of the first pixel cell 450 a. It should be noted that the operationof the circuit 450 would repeat the following steps for the operationfor pixel cells 450 b, 450 c, 450 d. Since the operation of theremaining pixels cells 450 b, 450 c, 450 d is essentially the same (withthe below noted exceptions), a detailed description of the operation ofpixels 450 b, 450 c, 450 d is not provided. FIG. 5 illustrates the rowselect signal ROW as being toggled high and low at certain instances.This control signal is active on low. It should be appreciated that therow select signal ROW could remain applied during all three time periodsT_(a), T_(b), T_(c), if desired.

The timing diagram illustrates three periods T_(a), T_(b), T_(c). Duringthe first time period T_(a), the row select signal ROW is applied to thegate of the row select transistor 414 (shown as being active low in FIG.5). It should be appreciated that FIG. 5 is an exemplary timing diagramand that it is immaterial whether a signal is illustrated as beingactive low or high in FIG. 5. All that is required to practice theinvention is for the illustrated signal to activate the component thesignal is controlling.

While the row select transistor is 414 is on, the floating diffusionregion 400 of the pixel circuit 450 is reset by asserting the resetcontrol signal RST (shown as being active low in FIG. 5). This causesthe array pixel supply voltage VAA_PIX to be applied to the floatingdiffusion region 400 (through the reset transistor 410). The DCG gatevoltage is applied to the capacitor 408 by activating the DCG transistor406 (pulsing of DCG). The reset signal voltage Vrst associated with thereset floating diffusion region 400 (as output by the source followertransistor 412 and activated row select transistor 414) is applied to acolumn line connected to sample and hold circuitry 265 (FIG. 9) and thensampled and held, for the first pixel cell 450 a, by the pulsing of areset sample and hold signal SHR.

During the second time period T_(b), charge accumulating in the firstphotosensor 450 a is transferred to the floating diffusion region 400when the first even transfer gate control signal TX_EVEN<0> is asserted(shown as being active low in FIG. 5) and activates the first transfertransistor 416 a. The pixel signal voltage Vsig1 associated with thefirst pixel cell's 450 a pixel signal charge stored in the floatingdiffusion region 400, as output by the source follower transistor 412and activated row select transistor 414, is applied to a column lineconnected to sample and hold circuitry 265 (FIG. 9) and then sampled andheld by the pulsing of a sample and hold pixel signal SHS.

To increase the charge storage capacity of the pixel cell 450 a, thefollowing operations are performed during the third time period T_(c).It should be noted that the following third time period T_(c) operationsmay be performed for every readout operation or only when needed toavoid the over capacity condition described above (i.e., when acontroller or image processor determines that the amount of incidentlight will result in the floating diffusion region 400 being saturated).

During the third time period T_(c), the dual conversion gain controlsignal DCG is applied (shown as being active low in FIG. 5). This causesthe DCG transistor 406 to become active, which connects the floatingdiffusion region 400 to the capacitor 408. The charge within thefloating diffusion region 400 also flows to and is stored in thecapacitor 408. The first even transfer gate control signal TX_EVEN<0> isapplied (shown as being active low in FIG. 5) to activate the firsttransfer transistor 416 a. The remaining excess charge from the firstphotosensor 401 a is stored in the floating diffusion region 400. Thenew pixel signal voltage Vsig2 associated with the excess pixel signalcharge stored in the floating diffusion region 400, as output by thesource follower transistor 412 and activated row select transistor 414,is applied to a column line connected to sample and hold circuitry 265(FIG. 9) and then sampled and held for the first pixel cell 450 a by thepulsing of a third sample and hold pixel signal (active high) SHD. Thethree sampled and held signals for pixel 450 a may then undergo acorrelated sampling operation to obtain the actual pixel signal levelfor each conversion gain (e.g., Vrst-Vsig1 and Vrst-Vsig2). Thesesignals may both be converted to digital signals and sent to the imageprocessor 280 discussed below, or only one of the two signals isselected, converted and sent to the image processor 280.

The operations are then repeated for the remaining pixels 450 b, 450 c,450 d. It should be noted that for the remaining pixels 450 b, 450 c,450 d, the same operations would occur except that the transfer gates416 b, 416 c, 416 d are controlled by the transfer gate control signalsTX_EVEN<1>, TX_ODD<0>, TX_ODD<1>, respectively.

It should be noted that if HDR transistors 418 a, 418 b, 418 c, 418 dare used in the pixel circuit 450, then the high dynamic range controlsignals HDR<0>, HDR<1>, for the first and second pixel rowsrespectively, would be applied throughout all three time periods T_(a),T_(b), T_(c) to ensure that the HDR transistors 418 a, 418 b, 418 c, 418d remain active during the readout operations. This prevents bloomingand other phenomena from occurring during the readout process bydraining some charge away from the photosensors 401 a, 401 b, 401 c, 401d.

In another exemplary embodiment according to the invention, a non-sharedpixel cell utilizes the capacitor-DCG combination. FIG. 6 illustrates atop down view of an imager pixel cell 550 according to this embodimentof the invention. FIG. 7 illustrates a circuit diagram for the pixelcell 550 illustrated in FIG. 6. FIGS. 6-7 depict a six transistortrumpet pixel having DCG transistor 506, HDR transistor 518, resettransistor 510, row select transistor 514, transfer transistor 516 andsource follower transistor 512. The readout trunk comprising the resettransistor 510, row select transistor 514, and source followertransistor 512 is located laterally to the left of the photosensor 501.The photosensor 501 is connected to the HDR transistor 518 and thetransfer transistor 516. The transfer transistor 516 is controllable bya transfer gate control signal TX. When the signal is generated, thetransfer transistor 516 transfers the charge collected by thephotosensor 501 to the floating diffusion region 500. The gate of thesource follower transistor 512 is connected to the floating diffusionregion 500. A source/drain terminal of the source follower transistor512 is connected to the array pixel supply voltage VAA_PIX. The rowselect transistor 514 is connected between the source followertransistor 512 and a column line 535.

The reset transistor 510 is connected between the floating diffusionregion 500 and array pixel supply voltage VAA_PIX. The reset transistor510 resets the floating diffusion region 500 before charge, istransferred by the transfer transistor 516. The DCG transistor 506 isbetween the floating diffusion region 500 and the capacitor 508. Thegate of the DCG transistor 506 functions as the bottom plate of thecapacitor 508. The gate of the DCG transistor 506 is connected toreceive a dual conversion gain control signal DCG. When the dualconversion gain control signal DCG is generated, the DCG transistor 506is activated which connects the floating diffusion region 500 to thecapacitor 508. As in the prior embodiment, one plate of the capacitor508 is also the gate of the DCG transistor 506 and receives voltage fromthe gate control signal for the DCG transistor 506.

FIG. 8 is a timing diagram illustrating an exemplary operation of thepixel cell 550 illustrated in FIGS. 6-7. The timing diagram illustratesthree periods T_(a), T_(b), T. During the first time period T_(a), therow select signal ROW is applied to the gate of the row selecttransistor 514 (shown as being active low in FIG. 8). It should beappreciated that FIG. 8 is an exemplary timing diagram and that it isimmaterial whether a signal is illustrated as being active low or highin FIG. 8. All that is required to practice the invention is for theillustrated control signal to activate the component the signal iscontrolling.

The floating diffusion region 500 of the pixel cell 550 is reset byasserting the reset control signal RST (shown as being active low inFIG. 8) at the same time. This causes the array pixel supply voltageVAA_PIX to be applied to the floating diffusion region 500 through thereset transistor 510. The reset signal voltage Vrst associated with thereset floating diffusion region 500, as output by the source followertransistor 512 and activated row select transistor 512, is applied to acolumn line connected to sample and hold circuitry 265 (FIG. 9) and thensampled and held by the pulsing of a reset sample and hold signal SHR(active high).

During the second time period T_(b), charge accumulating in thephotosensor 501 is transferred to the floating diffusion region 500 whenthe transfer gate control signal TX is asserted (shown as being activelow in FIG. 8) and activates the TX transistor 516. The pixel signalvoltage Vsig1 associated with the pixel signal charge stored in thefloating diffusion region 500, as output by the source followertransistor 512 and activated row select transistor 514, is applied to acolumn line connected to sample and hold circuitry 265 (FIG. 9) and thensampled and held by the pulsing of a sample and hold pixel signal SHS(active high).

To increase the charge storage capacity of the pixel cell 550, thefollowing operations are performed during the third time period T_(c).It should be noted that the following third time period T_(c) operationsmay be performed for every readout operation or only when needed toavoid the over capacity condition described above, i.e., when acontroller or image processor determines that the amount of incidentlight will result in the floating diffusion region being saturated.

During the third time period T_(c), the dual conversion gain controlsignal DCG is applied (shown as being active low in FIG. 8). This causesthe DCG transistor 506 to become active, which connects the floatingdiffusion region 500 to the capacitor 508. The full charge within thefloating diffusion region 500 also flows to and is stored in thecapacitor 508. The transfer gate control signal is applied (shown asbeing active low in FIG. 8) to activate the transfer transistor 516. Theremaining charge from the photosensor 501 is stored in the floatingdiffusion region 500. The new pixel signal voltage Vsig2 associated withthe pixel signal charge stored in the floating diffusion region 500, asoutput by the source follower transistor 512 and activated row selecttransistor 514, is applied to a column line connected to sample and holdcircuitry 265 (FIG. 9) and then sampled and held by the pulsing of athird sample and hold pixel signal SHD (active high). The three sampledand held signals may then undergo a correlated sampling operation toobtain the actual pixel signal level (e.g., (Vrst-Vsig1), (Vrst-Vsig2)).As in the first embodiment, both of the signals may be converted todigital signals and sent to the image processor 280, or only one of thetwo signals may be selected for conversion and sent to the imageprocessor 280.

It should be noted that if an HDR transistor 518 is used in the pixelcircuit 550, then the high dynamic range control signal HDR would beapplied throughout all three time periods T_(a), T_(b), T_(c) to ensurethat the HDR transistor 518 remains active during the charge integrationoperations. This photosensor prevents blooming from occurring.

The pixel structures described above may be formed in a substrate usingknown methods of transistor fabrication to form the transistors asdescribed herein in the embodied layouts illustrated in FIGS. 3 and 6.The photosensors 401 a, 401 b, 401 c, 401 d, 501 can be formed as anysuitable photosensitive structure for converting light photons intoelectrons (photo-charges). In a preferred embodiment, the photosensors401 a, 401 b, 401 c, 401 d, 501 are pinned photodiodes, however, theymay be formed as photogates, Schottky diodes, or as anotherphoto-sensing devices compatible with the invention.

FIG. 9 illustrates a block diagram of an exemplary CMOS imager 300having a pixel array 200 formed of pixel cells constructed in accordancewith the combined dual conversion gain transistor and capacitorembodiments described above (i.e., pixels 450 a, 450 b, 450 c, 450 d(FIG. 3), pixel 550 (FIG. 6)). Pixel array 200 comprises a plurality ofpixel cells arranged in a predetermined number of columns and rows (notshown). Attached to the array 200 is signal processing circuitry, asdescribed herein. The pixel cells of each row in array 200 can besimultaneously operated, and the pixel cells of each column areselectively output by respective column select lines. A plurality of rowand column lines are provided for the entire array 200. The row linesare selectively activated by a row driver 210 in response to row addressdecoder 220. The column select lines are selectively activated by acolumn driver 260 in response to column address decoder 270. Thus, a rowand column address is provided for each pixel cell.

The CMOS imager 300 is operated by the timing and control circuit 250,which controls address decoders 220, 270 for selecting the appropriaterow and column lines for pixel readout. The control circuit 250 alsocontrols the row and column driver circuitry 210, 260 such that theseapply driving voltages to the drive transistors of the selected row andcolumn lines. The pixel column signals, which typically include a pixelreset signal (Vrst), and the pixel image signals (Vsig1, Vsig2) are readby a sample and hold circuit 265 associated with the column drive 260. Asignal is produced by amplifier 267 for each pixel. The signal isdigitized by an analog-to-digital converter 275 (ADC). Theanalog-to-digital converter 275 supplies the digitized pixel signals toan image processor 280 which forms and outputs a digital image.

FIG. 10 shows a processor system 308, which includes an imager 300constructed in accordance with an embodiment of the invention. Theprocessor system 308 may be part of a digital camera or other imagingsystem. The imager 300 may receive control or other data from system308. System 308 includes a processor 302 having a central processingunit (CPU) for image processing, or other image handling operations. Theprocessor 302 communicates with various devices over a bus 304. Some ofthe devices connected to the bus 304 provide communication into and outof the system 308; an input/output (I/O) device 306 and an imagingdevice, i.e., imager 300, are such communication devices. Other devicesconnected to the bus 304 provide memory, for instance, a random accessmemory (RAM) 310 or a removable memory 315.

The processes and devices described above illustrate preferred methodsand typical devices of many that could be used and produced. The abovedescription and drawings illustrate embodiments, which achieve theobjects, features, and advantages of the present invention. However, itis not intended that the present invention be strictly limited to theabove-described and illustrated embodiments. Any modification of thepresent invention that comes within the spirit and scope of thefollowing claims should be considered part of the present invention.

1. An imaging device comprising: a plurality of photosensors; a shareddiffusion region for receiving charge generated by the photosensors; anda dual conversion gain element that can be selectively coupled to theshared diffusion region to increase a conversion gain of the shareddiffusion region.
 2. The imaging device of claim 1, further including:respective transfer transistors operable to couple each photosensor tothe shared diffusion region.
 3. The imaging device of claim 2, furtherincluding: respective transistors operable to drain charge from eachphotosensor.
 4. The imaging device of claim 1, further including: asource follower transistor, a gate of the source follower transistorbeing coupled to the shared diffusion region.
 5. The imaging device ofclaim 4, further including: a reset transistor operable to couple avoltage source to the shared diffusion region.
 6. The imaging device ofclaim 5, further including: a row select transistor operable to couplean output of the source follower transistor to an output of the imagingdevice.
 7. The imaging device of claim 1, wherein the dual conversiongain element comprises a transistor operable to couple a capacitor tothe shared diffusion region, a gate of the transistor forming one plateof the capacitor.
 8. The imaging device of claim 1, wherein theplurality of photosensors comprises four photosensors.
 9. The imagingdevice of claim 8, wherein two of the four photosensors are arranged ina first row of a pixel array, and another two of the four photosensorsare arranged in a second, adjacent row of the pixel array.
 10. Theimaging device of claim 1, wherein the photosensors are arranged in rowsand columns of a pixel array.
 11. An four-way shared pixel devicecomprising: first, second, third and fourth photosensors; a sharedstorage region for receiving charge generated by each of the first,second, third and fourth photosensors; and a dual conversion gainelement that can be selectively coupled to the shared storage region toincrease a conversion gain of the shared diffusion region.
 12. Thefour-way shared pixel device of claim 11, wherein the dual conversiongain element is operable to couple an additional charge storing elementto the shared storage region.
 13. The four-way shared pixel device ofclaim 12, wherein the dual conversion gain element comprises atransistor and the additional charge storing element comprises acapacitor.
 14. The four-way shared pixel device of claim 13, wherein agate of the transistor comprises one plate of the capacitor.
 15. Thefour-way shared pixel device of claim 11, further including respectiveelements operable to drain charge from each of the first, second, thirdand fourth photosensors.
 16. The four way shared pixel device of claim11, further including respective elements operable to selectively coupleeach of the first, second, third and fourth photosensors to the sharedstorage region.
 17. A method of operating an imaging device, the methodcomprising: resetting a shared diffusion region; sampling a resetvoltage level at the shared diffusion region; transferring chargeaccumulated in one of a plurality of photosensors to the shareddiffusion region; sampling a pixel signal voltage level at the shareddiffusion region; and activating a dual conversion gain element toincrease a conversion gain of the shared diffusion region.
 18. Themethod of claim 17, wherein the step of activating the dual conversiongain element is performed when an overcapacity condition is detectedduring sampling of the pixel signal voltage level.
 19. The method ofclaim 18, wherein the overcapacity condition is detected by comparingthe sampled pixel signal voltage level to a predetermined threshold. 20.The method of claim 17, wherein the dual conversion gain elementcomprises a transistor operable to couple a capacitor to the shareddiffusion region, a gate of the transistor forming one plate of thecapacitor, and the step of activating the dual conversion gain elementcomprises coupling the capacitor to the shared diffusion region.
 21. Themethod of claim 20, further comprising: sampling a conversion gainvoltage level at the shared diffusion region after activating the dualconversion gain element.
 22. The method of claim 21, further comprising:correlating the reset, pixel signal and conversion gain voltage levelsto determine an actual pixel signal voltage level.
 23. The method ofclaim 17, further including repeating the resetting, sampling resetvoltage level, transferring change, sampling pixel signal voltage anddual conversion gain element activating steps for each of the pluralityof photosensors.
 24. The method of claim 17, wherein the step oftransferring charge comprises activating a transfer transistorcorresponding to one of the photosensors to selectively couple thephotosensor to the diffusion region.
 25. The method of claim 24, whereinthe step of transferring charge further comprises receiving transfergate control signals to determine which of the photosensors toselectively couple to the diffusion region.